1) Field of the Invention
This invention relates to metal diaphragm valves, and, more particularly, so-called "metal-metal touch diaphragm valves", a type adapted for use in pipe arrangements through which ultra-clean fluids flow.
2) Description of the Prior Art
With the increasing micronization of circuit patterns in integrated circuit (IC) and large-scale integrated (LSI) chips in line with growing demand for their higher integration, increasing importance has been attached to the necessity of lessening, in manufacturing stages, as fine foreign matter present in cleaning fluid used to rinse semiconductor chips as possible. Such foreign matter is generally fine metal particles that are generated inside the valves in cleaning fluid pipe systems as a result of valve components sliding or colliding with each other. In order to minimize defects in the manufacturing of semiconductor chips, ultra-clean fluids with a purity level of around 1 ppb currently begin coming into universal use. In addition, there has been increasing demand on the use of such improved valves in pipe arrangements through which rinsing fluid are flowed, that can generate less foreign matters. To this aim, the valves should meet various requirements.
First, the valves must be of construction such as to minimize generation of metal particles. Secondly, the valves should withstand corrosion because corroded surfaces easily crumble letting rust go into the fluid. Furthermore, when the pipe arrangement has to change fluids, it is very essential to not allow any drop of the replaced fluid to remain in the piping before filling with a replacing fluid. There have been made attempts to satisfy the above-mentioned requirements.
One such a proposal was the use of a diaphragm valve. A typical diaphragm valve comprises a valve chamber containing a valve seat and a valve disk and a sheet made of plastics which serves to isolate the chamber from the outside.
However, conventional diaphragm valves have been found to pose problems. The plastic materials used in valve sheets tend to absorb some of the impurities contained in the cleaning fluids in their porous surface. A plastic sheet loaded with foreign matter would tend to another performance problem. In addition, such material does normally not has sufficient resistance against high temperatures and corrosion so that long operation can result in leakage and the generation of fine particles within the valve itself.
To solve the above difficulties, further improvements have been proposed. For example, U.S. Pat. No. 4,828,219 disclosed a metal diaphragm valve.
A metal diaphragm valve includes a metal diaphragm by which the interior of the valve casing is divided to define a valve chamber. Inlet and outlet holes are opened into the valve chamber. The valve chamber contains a valve seat that is situated between the inlet and outlet holes. A valve disk rests on the valve seat and is formed integrally with the diaphragm. A drive unit is mounted outside the valve chamber to drive the valve disk, thereby opening or closing the valve.
It has been discovered, however, that even the improvement is not without some problems. The metal diaphragms used in those metal diaphragm valves are simply shaped like flat disks. There is a major reason for the use of disk-shaped diaphragms; A thin metal diaphragm tends to produce irregular deformation, known as "waving", in its valve seat when it is about to come to full close. For a metal diaphragm valve to give the better performance, the metal diaphragm has to be shaped saucer like, with a deep concave in the center. The sufficiently deep concavity in a metal diaphragm would permit wide space between the valve seat and valve disk, which is situated below the diaphragm, enough to insure proper flow coefficient, Cv value. In other words, a flat disk-shaped diaphragm, while having to suffer much less from the impact of flapping than its saucer-like counterpart, have performance problems, even compared with other types of valve of the same dimension.
Moreover, the valve disk, which is located in the backside of a diaphragm and which is brought into contact with the valve seat at the time of valve closure, needs thorough polishing in that the surface is opposite the valve seat in order to remove the even finest scratchings, so that closure does not permit internal leakage. However, such perfect polishing finish does not only take a long time, but also involve very cumbersome labor. Also, because of the sheet flatness of the diaphragm surface, it is very difficult, if not impossible, to completely smooth out all the scratchings and scars from the disk's contacted face with the valve seat, enough to prevent possible leakage through the disk and seat.
Additionally, in operation, the valve stem is required to weigh on the diaphragm in such a manner to apply uniform pressure over the top surface of the valve seat, with the lower end surface of the stem held perfectly parallelly contacted with the valve seat, to insure leakproof closure. To this aim, it has to be designed so that the relation of the valve stem and the bonnet with the valve guide should be precise enough to allow the valve stem, in its descent onto the diaphragm, follow a path precisely perpendicular to the plane of the diaphragm.
Conventional metal diaphragm valves are not widely acceptable as they can easily allow the internal generation of fine metal particles and other impurities as a result of their structural failure to achieve perfect leakproof closure, as stated above.
The present invention, therefore, has been proposed to eliminate the drawback of difficulties with devices in the prior art.